You don't need this.

When using PKG_SEARCH_MODULE (line 29), if the package is found, you will get a variable called VRUI_INCLUDEDIR, which contains exactly what you ask in VRUI_INCLUDE_DIR.

You can remove line 27, and change what is currently line 31 to use ${VRUI_INCLUDEDIR} instead.

You will also need to remove the corresponding instruction in the readme.